Make groupBy preserve keys when grouping objects.

I've had this scenario when grouping objects such as
```javascript
    {
      1363402200: { "values": "1" },
      1363402201: { "values": "5" },
      1363402202: { "values": "17" }
    }
```
and using groupBy doesn't preserve the keys as this becomes
```javascript
_.groupBy(obj, function(v, k) { return k % 2; })

  {
    "0": [
      { "values": "1" },
      { "values": "17" }
    ],
    "1": [
      { "values": "5" }
    ]
  }
```
And the one I prefer is this which preserves the keys as I need to do additional operations on it.
```javascript
  {
    "0": {
      "1363402200": { "values": "1" },
      "1363402202": { "values": "17" }
    },
    "1": {
      "1363402201": { "values": "5" }
    }
  }
```

I'm not sure if this is a common use case as I've made "preserve keys" default instead of optional.